According to biologists and anthropologists who study our anatomy and our evolutionary history, humans are herbivores who are not well suited to eating meat.
Unlike natural carnivores, we are physically and psychologically unable to rip animals limb from limb and eat and digest their raw flesh. Even cooked meat is likely to cause human beings, but not natural carnivores, to suffer from food poisoning, heart disease, and other ailments.
People who pride themselves on being part of the human hunter tradition should take a second look at the story of human evolution. Prehistoric evidence indicates that humans developed hunting skills relatively recently and that most of our short, meat-eating past was spent scavenging and eating almost anything in order to survive; even then, meat was a tiny part of our caloric intake.
Humans lack both the physical characteristics of carnivores and the instinct that drives them to kill animals and devour their raw carcasses. Ask yourself: When you see dead animals on the side of the road, are you tempted to stop for a snack? Does the sight of a dead bird make you salivate? Do you daydream about killing cows with your bare hands and eating them raw? If you answered "no" to all of these questions, congratulations—you're a normal human herbivore—like it or not. Humans were simply not designed to eat meat.
I have put the summary chart at the beginning and again at the end of Dr. Herbert R. Mills excellent anatomical comparison of animals who eat the flesh of other animans with us to help you digest it easier.
Humans are most often described as "omnivores".
This classification is based on the "observation" that humans
generally eat a wide variety of plant and animal foods. However,
culture, custom and training are confounding variables when looking at
human dietary practices. Thus, "observation" is not the best
technique to use when trying to identify the most "natural"
diet for humans. While most humans are clearly "behavioral"
omnivores, the question still remains as to whether humans are
anatomically suited for a diet that includes animal as well as plant
foods.
A better and more objective technique is to look at
human anatomy and physiology. Mammals are anatomically and
physiologically adapted to procure and consume particular kinds of
diets. (It is common practice when examining fossils of extinct mammals
to examine anatomical features to deduce the animal's probable diet.)
Therefore, we can look at mammalian carnivores, herbivores
(plant-eaters) and omnivores to see which anatomical and physiological
features are associated with each kind of diet. Then we can look at
human anatomy and physiology to see in which group we belong.
Oral Cavity
Carnivores have a wide mouth opening in relation to
their head size. This confers obvious advantages in developing the
forces used in seizing, killing and dismembering prey. Facial
musculature is reduced since these muscles would hinder a wide gape, and
play no part in the animal's preparation of food for swallowing. In all
mammalian carnivores, the jaw joint is a simple hinge joint lying in the
same plane as the teeth. This type of joint is extremely stable and acts
as the pivot point for the "lever arms" formed by the upper
and lower jaws. The primary muscle used for operating the jaw in
carnivores is the temporalis muscle. This muscle is so massive in
carnivores that it accounts for most of the bulk of the sides of the
head (when you pet a dog, you are petting its temporalis muscles). The "angle"
of the mandible (lower jaw) in carnivores is small. This is because the
muscles (masseter and pterygoids) that attach there are of minor
importance in these animals. The lower jaw of carnivores cannot move
forward, and has very limited side-to-side motion. When the jaw of a
carnivore closes, the blade-shaped cheek molars slide past each other to
give a slicing motion that is very effective for shearing meat off bone.
The teeth of a carnivore are discretely spaced so as
not to trap stringy debris. The incisors are short, pointed and
prong-like and are used for grasping and shredding. The canines are
greatly elongated and dagger-like for stabbing, tearing and killing
prey. The molars (carnassials) are flattened and triangular with jagged
edges such that they function like serrated-edged blades. Because of the
hinge-type joint, when a carnivore closes its jaw, the cheek teeth come
together in a back-to-front fashion giving a smooth cutting motion like
the blades on a pair of shears.
The saliva of carnivorous animals does not contain
digestive enzymes. When eating, a mammalian carnivore gorges itself
rapidly and does not chew its food. Since proteolytic
(protein-digesting) enzymes cannot be liberated in the mouth due to the
danger of autodigestion (damaging the oral cavity), carnivores do not
need to mix their food with saliva; they simply bite off huge chunks of
meat and swallow them whole.
According to evolutionary theory, the anatomical
features consistent with an herbivorous diet represent a more recently
derived condition than that of the carnivore. Herbivorous mammals have
well-developed facial musculature, fleshy lips, a relatively small
opening into the oral cavity and a thickened, muscular tongue. The lips
aid in the movement of food into the mouth and, along with the facial
(cheek) musculature and tongue, assist in the chewing of food. In
herbivores, the jaw joint has moved to position above the plane of the
teeth. Although this type of joint is less stable than the hinge-type
joint of the carnivore, it is much more mobile and allows the complex
jaw motions needed when chewing plant foods. Additionally, this type of
jaw joint allows the upper and lower cheek teeth to come together along
the length of the jaw more or less at once when the mouth is closed in
order to form grinding platforms. (This type of joint is so important to
a plant-eating animal, that it is believed to have evolved at least 15
different times in various plant-eating mammalian species.) The angle of
the mandible has expanded to provide a broad area of attachment for the
well-developed masseter and pterygoid muscles (these are the major
muscles of chewing in plant-eating animals). The temporalis muscle is
small and of minor importance. The masseter and pterygoid muscles hold
the mandible in a sling-like arrangement and swing the jaw from
side-to-side. Accordingly, the lower jaw of plant-eating mammals has a
pronounced sideways motion when eating. This lateral movement is
necessary for the grinding motion of chewing.
The dentition of herbivores is quite varied depending
on the kind of vegetation a particular species is adapted to eat.
Although these animals differ in the types and numbers of teeth they
posses, the various kinds of teeth when present, share common structural
features. The incisors are broad, flattened and spade-like. Canines may
be small as in horses, prominent as in hippos, pigs and some primates
(these are thought to be used for defense) or absent altogether. The
molars, in general, are squared and flattened on top to provide a
grinding surface. The molars cannot vertically slide past one another in
a shearing/slicing motion, but they do horizontally slide across one
another to crush and grind. The surface features of the molars vary
depending on the type of plant material the animal eats. The teeth of
herbivorous animals are closely grouped so that the incisors form an
efficient cropping/biting mechanism, and the upper and lower molars form
extended platforms for crushing and grinding. The "walled-in"
oral cavity has a lot of potential space that is realized during eating.
These animals carefully and methodically chew their
food, pushing the food back and forth into the grinding teeth with the
tongue and cheek muscles. This thorough process is necessary to
mechanically disrupt plant cell walls in order to release the digestible
intracellular contents and ensure thorough mixing of this material with
their saliva. This is important because the saliva of plant-eating
mammals often contains carbohydrate-digesting enzymes which begin
breaking down food molecules while the food is still in the mouth.
Stomach and Small Intestine
Striking differences between carnivores and
herbivores are seen in these organs. Carnivores have a capacious simple
(single-chambered) stomach. The stomach volume of a carnivore represents
60-70% of the total capacity of the digestive system. Because meat is
relatively easily digested, their small intestines (where absorption of
food molecules takes place) are short -- about three to five or six
times the body length. Since these animals average a kill only about
once a week, a large stomach volume is advantageous because it allows
the animals to quickly gorge themselves when eating, taking in as much
meat as possible at one time which can then be digested later while
resting. Additionally, the ability of the carnivore stomach to secrete
hydrochloric acid is exceptional. Carnivores are able to keep their
gastric pH down around 1-2 even with food present. This is necessary to
facilitate protein breakdown and to kill the abundant dangerous bacteria
often found in decaying flesh foods.
Because of the relative difficulty with which various
kinds of plant foods are broken down (due to large amounts of
indigestible fibers), herbivores have significantly longer and in some
cases, far more elaborate guts than carnivores. Herbivorous animals that
consume plants containing a high proportion of cellulose must "ferment"
(digest by bacterial enzyme action) their food to obtain the nutrient
value. They are classified as either "ruminants" (foregut
fermenters) or hindgut fermenters. The ruminants are the plant-eating
animals with the celebrated multiple-chambered stomachs. Herbivorous
animals that eat a diet of relatively soft vegetation do not need a
multiple-chambered stomach. They typically have a simple stomach, and a
long small intestine. These animals ferment the difficult-to-digest
fibrous portions of their diets in their hindguts (colons). Many of
these herbivores increase the sophistication and efficiency of their GI
tracts by including carbohydrate-digesting enzymes in their saliva. A
multiple-stomach fermentation process in an animal which consumed a diet
of soft, pulpy vegetation would be energetically wasteful. Nutrients and
calories would be consumed by the fermenting bacteria and protozoa
before reaching the small intestine for absorption. The small intestine
of plant-eating animals tends to be very long (greater than 10 times
body length) to allow adequate time and space for absorption of the
nutrients.
Colon
The large intestine (colon) of carnivores is simple
and very short, as its only purposes are to absorb salt and water. It is
approximately the same diameter as the small intestine and,
consequently, has a limited capacity to function as a reservoir. The
colon is short and non-pouched. The muscle is distributed throughout the
wall, giving the colon a smooth cylindrical appearance. Although a
bacterial population is present in the colon of carnivores, its
activities are essentially putrefactive.
In herbivorous animals, the large intestine tends to
be a highly specialized organ involved in water and electrolyte
absorption, vitamin production and absorption, and/or fermentation of
fibrous plant materials. The colons of herbivores are usually wider than
their small intestine and are relatively long. In some plant-eating
mammals, the colon has a pouched appearance due to the arrangement of
the muscle fibers in the intestinal wall. Additionally, in some
herbivores the cecum (the first section of the colon) is quite large and
serves as the primary or accessory fermentation site.
What About Omnivores?
One would expect an omnivore to show anatomical
features which equip it to eat both animal and plant foods. According to
evolutionary theory, carnivore gut structure is more primitive than
herbivorous adaptations. Thus, an omnivore might be expected to be a
carnivore which shows some gastrointestinal tract adaptations to an
herbivorous diet.
This is exactly the situation we find in the Bear,
Raccoon and certain members of the Canine families. (This discussion
will be limited to bears because they are, in general, representative of
the anatomical omnivores.) Bears are classified as carnivores but are
classic anatomical omnivores. Although they eat some animal foods, bears
are primarily herbivorous with 70-80% of their diet comprised of plant
foods. (The one exception is the Polar bear which lives in the frozen,
vegetation poor arctic and feeds primarily on seal blubber.) Bears
cannot digest fibrous vegetation well, and therefore, are highly
selective feeders. Their diet is dominated by primarily succulent lent
herbage, tubers and berries. Many scientists believe the reason bears
hibernate is because their chief food (succulent vegetation) not
available in the cold northern winters. (Interestingly, Polar bears
hibernate during the summer months when seals are unavailable.)
In general, bears exhibit anatomical features
consistent with a carnivorous diet. The jaw joint of bears is in the
same plane as the molar teeth. The temporalis muscle is massive, and the
angle of the mandible is small corresponding to the limited role the
pterygoid and masseter muscles play in operating the jaw. The small
intestine is short ( less than five times body length) like that of the
pure carnivores, and the colon is simple, smooth and short. The most
prominent adaptation to an herbivorous diet in bears (and other "anatomical"
omnivores) is the modification of their dentition. Bears retain the
peg-like incisors, large canines and shearing premolars of a carnivore;
but the molars have become squared with rounded cusps for crushing and
grinding. Bears have not, however, adopted the flattened, blunt nails
seen in most herbivores and retain the elongated, pointed claws of a
carnivore.
An animal which captures, kills and eats prey must
have the physical equipment which makes predation practical and
efficient. Since bears include significant amounts of meat in their
diet, they must retain the anatomical features that permit them to
capture and kill prey animals. Hence, bears have a jaw structure,
musculature and dentition which enable them to develop and apply the
forces necessary to kill and dismember prey even though the majority of
their diet is comprised of plant foods. Although an herbivore-style jaw
joint (above the plane of the teeth) is a far more efficient joint for
crushing and grinding vegetation and would potentially allow bears to
exploit a wider range of plant foods in their diet, it is a much weaker
joint than the hinge-style carnivore joint. The herbivore-style jaw
joint is relatively easily dislocated and would not hold up well under
the stresses of subduing struggling prey and/or crushing bones (nor
would it allow the wide gape carnivores need). In the wild, an animal
with a dislocated jaw would either soon starve to death or be eaten by
something else and would, therefore, be selected against. A given
species cannot adopt the weaker but more mobile and efficient
herbivore-style joint until it has committed to an essentially
plant-food diet lest it risk jaw dislocation, death and ultimately,
extinction.
What About Me?
The human gastrointestinal tract features the
anatomical modifications consistent with an herbivorous diet. Humans
have muscular lips and a small opening into the oral cavity. Many of the
so-called "muscles of expression" are actually the muscles
used in chewing. The muscular and agile tongue essential for eating, has
adapted to use in speech and other things. The mandibular joint is
flattened by a cartilaginous plate and is located well above the plane
of the teeth. The temporalis muscle is reduced. The characteristic "square
jaw" of adult males reflects the expanded angular process of the
mandible and the enlarged masseter/pterygoid muscle group. The human
mandible can move forward to engage the incisors, and side-to-side to
crush and grind.
Human teeth are also similar to those found in other
herbivores with the exception of the canines (the canines of some of the
apes are elongated and are thought to be used for display and/or
defense). Our teeth are rather large and usually abut against one
another. The incisors are flat and spade-like, useful for peeling,
snipping and biting relatively soft materials. The canines are neither
serrated nor conical, but are flattened, blunt and small and function
Like incisors. The premolars and molars are squarish, flattened and
nodular, and used for crushing, grinding and pulping noncoarse foods.
Human saliva contains the carbohydrate-digesting
enzyme, salivary amylase. This enzyme is responsible for the majority of
starch digestion. The esophagus is narrow and suited to small, soft
balls of thoroughly chewed food. Eating quickly, attempting to swallow a
large amount of food or swallowing fibrous and/or poorly chewed food
(meat is the most frequent culprit) often results in choking in humans.
Man's stomach is single-chambered, but only
moderately acidic. (Clinically, a person presenting with a gastric pH
less than 4-5 when there is food in the stomach is cause for concern.)
The stomach volume represents about 21-27% of the total volume of the
human GI tract. The stomach serves as a mixing and storage chamber,
mixing and liquefying ingested foodstuffs and regulating their entry
into the small intestine. The human small intestine is long, averaging
from 10 to 11 times the body length. (Our small intestine averages 22 to
30 feet in length. Human body size is measured from the top of the head
to end of the spine and averages between two to three feet in length in
normal-sized individuals.)
The human colon demonstrates the pouched structure
peculiar to herbivores. The distensible large intestine is larger in
cross-section than the small intestine, and is relatively long. Man's
colon is responsible for water and electrolyte absorption and vitamin
production and absorption. There is also extensive bacterial
fermentation of fibrous plant materials, with the production and
absorption of significant amounts of food energy (volatile short-chain
fatty acids) depending upon the fiber content of the diet. The extent to
which the fermentation and absorption of metabolites takes place in the
human colon has only recently begun to be investigated.
In conclusion, we see that human beings have the
gastrointestinal tract structure of a "committed" herbivore.
Humankind does not show the mixed structural features one expects and
finds in anatomical omnivores such as bears and raccoons. Thus, from
comparing the gastrointestinal tract of humans to that of carnivores,
herbivores and omnivores we must conclude that humankind's GI tract is
designed for a purely plant-food diet.
Summary
Facial Muscles
CARNIVORE: Reduced to allow wide mouth gape
HERBIVORE: Well-developed
OMNIVORE: Reduced
HUMAN: Well-developed
Jaw Type
CARNIVORE: Angle not expanded
HERBIVORE: Expanded angle
OMNIVORE: Angle not expanded
HUMAN: Expanded angle
Jaw Joint Location
CARNIVORE: On same plane as molar teeth
HERBIVORE: Above the plane of the molars
OMNIVORE: On same plane as molar teeth
HUMAN: Above the plane of the molars
Jaw Motion
CARNIVORE: Shearing; minimal side-to-side motion
HERBIVORE: No shear; good side-to-side, front-to-back
OMNIVORE: Shearing; minimal side-to-side
HUMAN: No shear; good side-to-side, front-to-back
Major Jaw Muscles
CARNIVORE: Temporalis
HERBIVORE: Masseter and pterygoids
OMNIVORE: Temporalis
HUMAN: Masseter and pterygoids
Mouth Opening vs. Head Size
CARNIVORE: Large
HERBIVORE: Small
OMNIVORE: Large
HUMAN: Small
Teeth: Incisors
CARNIVORE: Short and pointed
HERBIVORE: Broad, flattened and spade shaped
OMNIVORE: Short and pointed
HUMAN: Broad, flattened and spade shaped
Teeth: Canines
CARNIVORE: Long, sharp and curved
HERBIVORE: Dull and short or long (for defense), or none
OMNIVORE: Long, sharp and curved
HUMAN: Short and blunted
Teeth: Molars
CARNIVORE: Sharp, jagged and blade shaped
HERBIVORE: Flattened with cusps vs complex surface
OMNIVORE: Sharp blades and/or flattened
HUMAN: Flattened with nodular cusps
Chewing
CARNIVORE: None; swallows food whole
HERBIVORE: Extensive chewing necessary
OMNIVORE: Swallows food whole and/or simple crushing
HUMAN: Extensive chewing necessary
Saliva
CARNIVORE: No digestive enzymes
HERBIVORE: Carbohydrate digesting enzymes
OMNIVORE: No digestive enzymes
HUMAN: Carbohydrate digesting enzymes
Stomach Type
CARNIVORE: Simple
HERBIVORE: Simple or multiple chambers
OMNIVORE: Simple
HUMAN: Simple
Stomach Acidity
CARNIVORE: Less than or equal to pH 1 with food in
stomach
HERBIVORE: pH 4 to 5 with food in stomach
OMNIVORE: Less than or equal to pH 1 with food in stomach
HUMAN: pH 4 to 5 with food in stomach
Stomach Capacity
CARNIVORE: 60% to 70% of total volume of digestive
tract
HERBIVORE: Less than 30% of total volume of digestive tract
OMNIVORE: 60% to 70% of total volume of digestive tract
HUMAN: 21% to 27% of total volume of digestive tract
Length of Small Intestine
CARNIVORE: 3 to 6 times body length
HERBIVORE: 10 to more than 12 times body length
OMNIVORE: 4 to 6 times body length
HUMAN: 10 to 11 times body length
Colon
CARNIVORE: Simple, short and smooth
HERBIVORE: Long, complex; may be sacculated
OMNIVORE: Simple, short and smooth
HUMAN: Long, sacculated
Liver
CARNIVORE: Can detoxify vitamin A
HERBIVORE: Cannot detoxify vitamin A
OMNIVORE: Can detoxify vitamin A
HUMAN: Cannot detoxify vitamin A
Kidney
CARNIVORE: Extremely concentrated urine
HERBIVORE: Moderately concentrated urine
OMNIVORE: Extremely concentrated urine
HUMAN: Moderately concentrated urine
Nails
CARNIVORE: Sharp claws
HERBIVORE: Flattened nails or blunt hooves
OMNIVORE: Sharp claws
HUMAN: Flattened nails
